1. Field of the Invention
The present invention relates to an image capture device, and more particularly, to an image capture device using focal sweep technique to facilitate an automatic focus searching procedure and a related method thereof.
2. Description of the Prior Art
Automatic focus searching (AF) techniques are commonly applied in cameras. A traditional automatic focus searching method for a camera is to control the lens to move in various lens positions and measure sharpness of an image with respect to each of the lens positions. When peak sharpness is found through the above focus searching operation, the camera will output an image based on the focus located at the lens position corresponding to the found peak sharpness. In the above method, the lens of the camera must be moved to an initial lens position to measure a first sharpness value with respect to the initial lens position, and is then moved to a next lens position to measure a second sharpness value, and so on. When measuring sharpness for a next lens position, the camera will expose pixels of the sensor and measure the sharpness only when the lens is already at the lens position and remains settled.
For each camera, it is desirable to have a fast automatic focus searching. However, even when the camera is operated at high frame rate, a waiting procedure is inevitable. It usually takes time for an automatic focus searching run normally composed of several stepping cycles of waiting for the lens to settle on a certain location, exposing the pixels of the sensor for a certain exposure time, reading out pixel data from the sensor, and calculating sharpness information of the readout pixel data before the optimal focus can be determined. That is, even if the image sensor can output frames at a high rate, the automatic focus searching is still relatively slow due to the stepping cycles.
Please refer to FIG. 1, which is a diagram showing timing sequence of procedures employed in a conventional automatic focus searching. FIG. 1 illustrates timing charts of frames, exposing commands, sharpness measurements, and voice coil motor (VCM) commands from top to bottom. Each exposing command is executed for a corresponding frame in advance. After exposure, pixel data are read out so that the camera can start to measure the sharpness information for the frame. The lens is required to move and converge to a target position before the sharpness is measured. For example, at the beginning of the frame i, a corresponding VCM command is issued to move/drive the lens. Suppose the lens can be settled in an interval of one frame, an exposing command will be issued at the end of the frame i such that the sensor is allowed to start exposure. Afterwards, the camera can thus measure the sharpness information from the readout pixel data of frame i+1. This waiting makes the entire automatic focus searching procedure even less efficient.
One of simplest method to shorten the time required by the automatic focus searching is to increase the output frame rate. A high output frame rate normally imposes constraint on affordable longest exposure time and thus reduces practical exposure time. However, for the high output frame rate in a low-lighted condition, it has to level up gain/ISO (International Standards Organization) sensitivity to maintain the brightness of the images, while the exposure time is short. Unfortunately, it also deteriorates the signal-to-noise ratio (SNR) of images. Hence, there is a need to provide a novel method for improving the conventional automatic focus searching procedure.